In Harper Lee's To Kill a Mockingbird, after the children have been attacked and rescued, and Bob Ewell's actions are realized along with the discovery of his dead body, Atticus at first believes that Jem killed Ewell. In fact, he and Heck Tate have an argument because Atticus believes that Heck is simply trying to save Jem from any trouble by making up a tale about how Ewell died. Atticus is the moral compass of this story: he is ethical (knows what's right) and moral (insists upon doing the right thing). He tells Heck that he (Atticus) must set a good example by acting upon the truth or he will have betrayed his children's trust.

Heck explains that he isn't worried about Jem because Jem did not kill Bob Ewell. Heck infers that it was Boo Radley (who forever watches the children he so loves) that saved Scout and Jem from Ewell's attempt to murder them both. In Chapter Thirty, Heck shares his reasoning with Atticus along with his predictions of what will happen if the town discovers that Boo is the hero:

[It isn’t] against the law for a citizen to do his utmost to prevent a crime from being committed, which is exactly what he did, but maybe you'll say it's my duty to tell the town all about it and not hush it up. Know what'd happen then? All the ladies in Maycomb includin' my wife'd be knocking on his door bringing angel food cakes. To my way of thinkin', Mr. Finch…draggin' him with his shy ways into the limelight—to me, that's a sin. It's a sin and I'm not about to have it on my head.

Boo Radley is rarely seen, coming out only at night. He is like the mockingbird that Atticus and Miss Maudie tell the children must be protected: for Boo has been abused by his family and is now little more than a shadow of a man—a child in many ways. However, the children's preoccupation with Boo has endeared them to the man: it is for this reason that Boo was watching the night Jem and Scout returned late from the school play.

In this case, justice is not as clear-cut now for Atticus: not a man to lie, he is also compassionate enough to want to protect Boo Radley, as he has been throughout the story. However, now he also owes the lives of his children to this solitary man. Atticus' sense of justice, the reader can infer, demands that he protect the defenseless Boo Radley from the well-meaning and potentially terrifying attentions of the townspeople; and so he explains to Scout that Bob Ewell fell on his knife and died, excluding any mention of the presence of Boo Radley that night.